Sir Menzies Campbell made an unlikely sort of Robin Hood yesterday, dressed in a tailored blue suit and salmon-pink silk tie, as he spoke of taking from the very rich to help the poor and robbing the low-cost flyers of Nottingham to save the Sherwood greenery. Standing in front of his MPs at an event in Westminster intended to lend focus to what has so far been a blurred leadership, Sir Menzies raised the prospect of radical tax reform as a route to "a liberal country". The ambition is fine but the reality, at least on the partial details set out yesterday, falls short. He talked of overcoming inequality and failing public services but the political pitch was aimed squarely at his party's supporters in middle Britain: a £20bn income tax cut that will do nothing for the very poor.

Almost 100 days since he replaced Charles Kennedy at his party's head, Sir Menzies faces the charge that he is drifting to the right. At the last general election the Lib Dems supported two progressive moves on taxation: a local income tax and a 50p top rate for high earners - described yesterday by Sir Menzies "as an important symbol that we are a redistributive party". Yesterday he confirmed that the 50p rate is to go and also announced a 2p cut in the basic rate and a raising of thresholds that together give most to the middle classes. That leaves local income tax as the party's redistributive flagship, but its effect will be obliterated by the national income tax cuts.

Yesterday's plans also leave the party on the hunt for £20bn in extra tax. It does not claim to have come up with the answers yet: there is more to come in July, when its tax commission reports. But even doubling air passenger duty and the climate change levy will only raise £2bn; doubling capital gains tax £3.8bn. The hole is deep and they will struggle to fill it with taxes on polluters and the very rich, despite Sir Menzies' claims to the contrary.

Behind the nitpicking, however, lies a straightforward question about what sort of party the Liberal Democrats want to become. What - and whom - are they for? Hunting for a maturity justified by the 6 million votes won at the last election, the party risks blunting its brave edge, especially with the loss of the 50p top rate. Environmental taxation, if followed with the rigour promised yesterday, might bring back some sharpness. Sir Menzies wants a reputation for clear thinking and authority. He will need to fill in yesterday's blanks and make it clear his party cannot cherry-pick his plans if they are to be seen as something more than a bribe for the votes of middle England.